As a battery charging system for motorcycles of related art, a short FET REG/REC including a permanent-magnet three-phase alternating-current power generator (ACG) is used (see, for example, Patent Document 1).
For example, regarding a battery charger shown in FIG. 4, diodes D1 to D3 rectify currents to charge a battery Batt when the battery Batt is charged. While the battery Batt is not charged, MOS transistors M1 to M3 are turned on, and the currents return to the three-phase alternating-current power generator ACG.
The MOS transistor M1 is connected in series to the diode D1. The MOS transistor M2 is connected in series to the diode D2. The MOS transistor M3 is connected in series to the diode D3. The MOS transistor M1 is turned on and off under control of the gate controller CONT1. The MOS transistor M2 is turned on and off under control of the gate controller CONT2. The MOS transistor M3 is turned on and off under control of the gate controller CONT3.
In other words, while the battery Batt is charged, as a result of zero-cross detection by comparators (CMP1, CMP2, and CMP3), the gate controllers perform synchronous rectification in which each of the gate controllers turns off the corresponding MOS transistor when an AC voltage is positive, and turns on the corresponding MOS transistor when the AC voltage is negative.
On the other hand, when the battery Batt becomes fully charged, the gate controllers turn on the corresponding FETs. Then, phases (U, V, and W) of the ACG are shorted through the respective FETs. Thus, the charging of the battery Batt is terminated, and the battery charger enters a non-charging state.